The present invention relates to the field of power amplifiers. More particularly, the invention relates to a method and apparatus for improving the efficiency of power amplifiers operating under large peak-to-average ratios, while eliminating the need for clipping signals having large peak amplitudes.
Modern communication systems, such as cellular systems employ power amplifiers in their basestations, in order to communicate with subscribers that are distributed in cells. These power amplifiers that are required to amplify Radio Frequency (RF) signals, such as signals used in communication systems that are required to transmit multiple signals, simultaneously. Multiple signals should be transmitted, for example, due to multiple users sharing the same frequency band, such as cellular systems that are operated in Code Division Multiple Access (CDMA) regimes. Another communication method that requires simultaneous transmissions employ, for example, a modulation format known as xe2x80x9cmulti-tonexe2x80x9d, or Orthogonal Frequency Division Multiplexing (OFDM), in which the signal from a single user is first subdivided. Each subdivision is then modulated by a multiplicity of staggered subcarriers. The modulated subcarriers are then added up, thus causing large peak excursions.
Conventional RF amplifiers required to simultaneously amplify RF signals that have large peak-to-average ratios, are costly and relatively inefficient (consuming much DC power). The reason for such inefficiency is that a power amplifier becomes efficient only during the occurrence of the peaks, i.e., when the instantaneous power output is large. However, during most of the time, the power output is only a small fraction of the power drain from the Direct Current (DC) power supply, resulting in low efficiency.
In order to reduce the average power loss, communication system designers use conventional techniques for reducing the peak to average ratio, based on clipping of the signal peaks. xe2x80x9cKeeping noise mitigation for ODFM by decision-aided reconstructionxe2x80x9d to Kim et al, IEEE Communications Letters, Vol. 3, No. 1, January 1999 and xe2x80x9cDesign considerations for multicarrier CDMA base station power amplifiersxe2x80x9d, to J. S. Kenney et al, Microwave Journal, February 1999, describe such techniques, which treat OFDM and multicarrier communications. It is also explained there that clipping considerably increases the undesired error rate of the system, and in some cases cause a partial spectral re-growth. Considerable effort is directed to mitigate the increase in the error rate while increasing the amount of clipping.
xe2x80x9cConsiderations on applying OFDM in a highly efficient power amplifierxe2x80x9d to W. Liu et al, IEEE transactions on circuits and systems, Vol. 46, No. 11, November 1999, relates to the classical Envelope Elimination and Reconstruction (EER) for OFDM. xe2x80x9cDevice and circuit approaches for next-generation wireless communicationsxe2x80x9d to P. Asbeck et al, Microwave Journal, February 1999, discloses similar features EER for OFDM, with sundry modifications for multicarrier transmission. However, all the above references depend on continuously varying power supplies for the envelope reconstruction or emphasis, which is difficult to achieve at large bandwidths and large peak to average ratios. Furthermore, EER techniques are mostly used for low-frequency modulation.
All the methods described above have not yet provided satisfactory solutions to the problem of improving the efficiency of power amplifiers operated under large peak-to-average ratios, while eliminating the need for clipping signals having large peak amplitudes.
It is an object of the present invention to provide a method and apparatus for improving the efficiency of power amplifiers operated under large peak-to-average ratios, while eliminating the need for clipping signals.
It is another object of the present invention to provide a method and apparatus for improving the efficiency of power amplifiers operated under large peak-to-average ratios, while eliminating spectral re-growth of unwanted sidebands.
It is still another object of the present invention to provide a method and apparatus for expanding the dynamic range of power amplifiers operated under large peak-to-average ratios.
Other objects and advantages of the invention will become apparent as the description proceeds.
The present invention is directed to a method for improving the efficiency and the dynamic range of a power amplifier operated with signals having a large peak-to-average ratio. A reference level is determined, above which at least a portion of the magnitude of an input signal being a modulated signal that is input to the power amplifier, or a baseband waveform that is used to generate the modulated signal, is defined as an excess input signal. The magnitude of the input signal is continuously sampled, for detecting an excess input signal. A lower level of operating voltage is supplied to the power amplifier, if no excess input signal is detected. The lower level of operating voltage is sufficient to effectively amplify input signals having a magnitude below the reference level. A higher level of operating voltage is supplied to the power amplifier, whenever an excess input signal is detected. The higher level of operating voltage is sufficient to effectively amplify input signals having a magnitude above the reference level.
Preferably, an automatic gain control circuit is coupled to the input of the power amplifier, in order to control the magnitude of the input signal(s) prior to amplification. Whenever an excess input signal is detected, the excess input signal is sampled. Changes in the gain of the power amplifier during the presence of the excess input signal are compensated by controlling the gain of the automatic gain control circuit, according to the samples of the excess input signal.
The level of operating voltage is supplied to the power amplifier by a lower voltage source for feeding the power amplifier whenever no excess input signal is detected and a higher voltage source for feeding the power amplifier whenever an excess input signal is detected. The voltage supply contact of the power amplifier is connected to the lower voltage source through a first variable impedance, and may be connected to the higher voltage source through a second variable impedance. Whenever no excess input signal is detected, the first and the second variable impedances may be simultaneously controlled to be in an appropriate low, and highest impedance states, respectively. Whenever an excess input signal is detected, the first and the second variable impedances may be simultaneously controlled to be in their highest and an appropriate low impedance states, respectively.
Preferably, the level of operating voltage is supplied to the power amplifier by using another voltage source for feeding the power amplifier whenever an excess input signal is detected. The voltage supply contact of the power amplifier is connected to the first voltage source through a variable impedance, and to the another voltage source through a voltage amplifier. The variable impedance can present a low resistannce to DC and high impedance for rapidly varying pulses. The variable impedance is allowed to reach an appropriate low resistance whenever no excess input signal is detected, and its high impedance whenever an excess input signal is detected. The voltage amplifier is allowed to supply a voltage level that is higher than the voltage of the first voltage source, to the voltage supply contact of the power amplifier, whenever an excess input signal is detected. At least one of the variable impedances may be an inductor or a diode, or a controllable impedance, such as a bipolar transistor or a FET.
Preferably, levels of operating voltage, supplied to the power amplifier, are normalized to corresponding predetermined levels of the excess input signal. The level of operating voltage supplied to the power amplifier sampled and an error signal is generated by comparing between the sampled level with the excess input signal. The error signal is used to operate a negative feedback loop for accurately controlling the operating voltage supplied to the power amplifier.
Alternatively, levels of operating voltage, supplied to the power amplifier, are normalized to corresponding predetermined levels of RF output signals, amplified by the power amplifier. The level of RF output signals, amplified by the power amplifier is sampled and an error signal is generated by comparing between the sampled level with the excess input signal. The error signal is used for operating a negative feedback loop for accurately controlling the operating voltage supplied to the power amplifier.
According to another aspect of the invention, the level of DC voltage, supplied to the power amplifier, is controlled, using the baseband waveform. A baseband signal source outputs the baseband waveform into a modulator, that generates a modulated input signal which is fed into the power amplifier. A reference level, above which at least a portion of the baseband waveform, is defined as an excess baseband signal is determined. The magnitude of the baseband waveform is continuously sampled, for detecting an excess baseband signal. A lower level of operating voltage is supplied to the power amplifier, if no excess baseband signal is detected. The lower level of operating voltage is sufficient to effectively amplify input signals, modulated with a baseband waveform of a magnitude below the reference level. A higher level of operating voltage is supplied to the power amplifier, whenever an excess baseband signal is detected. The higher level of operating voltage is sufficient to effectively amplify input signals that are modulated with a baseband waveform of a magnitude above the reference level.
The present invention is also directed to an apparatus for improving the efficiency and the dynamic range of a power amplifier operated with signals having a large peak-to-average ratio. The apparatus comprises a sampling circuit for continuously sampling the magnitude of an input signal, which may be a modulated signal that input to the power amplifier, or of a baseband waveform that is used to generate the modulated signal. The sampling circuit detects an excess input signal according to a predetermined reference level, above which at least a portion of the input signal is defined as an excess input signal; a power supply for indirectly supplying an operating voltage to the power amplifier; and a control circuit that operates in combination with the power supply, for causing the power supply to supply a lower level of operating voltage that is sufficient to effectively amplify input signals of a magnitude below the reference level, to the power amplifier, if no excess input signal is detected, and to supply a higher level of operating voltage that is sufficient to effectively amplify input signals of a magnitude above the reference level, to the power amplifier, whenever an excess input signal is detected.
The apparatus may further comprise:
a) an automatic gain control circuit, coupled to the input of the power amplifier, for controlling the magnitude of the input signal(s) prior to amplification;
b) circuitry for sampling the excess input signal; and
c) a control circuitry, for compensating changes in the gain of the power amplifier during the presence of the excess input signal by controlling the gain of the automatic gain control circuit, according to the samples of the excess input signal.
The apparatus may comprise:
a) a lower voltage source for feeding the power amplifier whenever no excess input signal is detected;
b) a higher voltage source for feeding the power amplifier whenever an excess input signal is detected;
c) a first variable impedance connected between the voltage supply input of the power amplifier and the lower voltage source;
d) a second variable impedance connected between the voltage supply input of the power amplifier and the higher voltage source; and
e) a control circuit for simultaneously controlling the first and the second variable impedances to be in an appropriate low, and highest impedance states, respectively, whenever no excess input signal is detected, and to be in their highest, and an appropriate low impedance states, respectively, whenever an excess input signal is detected.
Preferably, the apparatus comprises:
a) a first voltage source for feeding the power amplifier whenever no excess input signal is detected;
b) another voltage source for feeding the power amplifier whenever an excess input signal is detected;
c) a variable impedance connected between the voltage supply contact of the power amplifier and the first voltage source, the variable impedance being capable of presenting a low resistance to DC and high impedance for rapidly varying pulses;
d) a voltage amplifier connected between the voltage supply contact of the power amplifier and the another voltage source, for supplying a voltage level, being higher than the voltage of the first voltage source, to the voltage supply contact of the power amplifier;
e) a control circuit, for controlling the voltage amplifier to supply a voltage level, being higher than the voltage of the first voltage source, to the voltage supply contact of the power amplifier, whenever no excess input signal is detected, and if the variable impedance is a controllable impedance:
for controlling the variable impedance to reach its high impedance value whenever an excess input signal is detected, and to reach an appropriate low resistance whenever no excess input signal is detected.
At least one of the variable impedances of the apparatus may be an inductor or a diode, or a controllable impedance, such as a bipolar transistor or a FET. The apparatus may further comprise:
a) a sampling circuit for sampling the level of operating voltage supplied to the power amplifier;
b) a comparator for generating an error signal by comparing between the sampled level of operating voltage supplied to the power amplifier, with the level of the excess input signal; and
c) a negative feedback loop for accurately controlling the operating voltage supplied to the power amplifier, by using the error signal.
The apparatus may further comprise:
a) a sampling circuit for sampling the level of RF output signals, amplified by the power amplifier;
b) a comparator for generating an error signal by comparing between the sampled level of RF output signals, amplified by the power amplifier, with the level of the excess input signal; and
c) a negative feedback loop for accurately controlling the operating voltage supplied to the power amplifier, by using the error signal.
Alternatively, the apparatus may further comprise:
a) a modulator for generating a modulated signal that is input to the power amplifier;
b) a baseband signal source for generating a baseband waveform that that is input to the modulator;
c) a sampling circuit for continuously sampling the magnitude of the baseband waveform, for detecting an excess input signal according to a predetermined reference level, above which at least a portion of the baseband waveform is defined as an excess baseband signal;
d) a power supply for indirectly supplying an operating voltage to the power amplifier; and
e) a control circuit, operating in combination with the power supply, for causing the power supply to supply a lower level of operating voltage being sufficient to effectively amplify input signals having a magnitude below the reference level, to the power amplifier, if no excess input signal is detected, and to supply a higher level of operating voltage being sufficient to effectively amplify input signals having a magnitude above the reference level, to the power amplifier, whenever an excess input signal is detected.